1. Field of the Invention
This invention generally relates to a method for recording an image on a transfer medium by first forming an image on an imaging surface and then having the image transferred to a transfer medium and in particular to a transfer-type electrostatic recording method using an endless imaging belt.
2. Description of the Prior Art
FIG. 1 shows a typical electrostatic recording system of the image transfer type. As shown, the system includes an endless imaging belt 1 which comprises a supporting layer 1a, an electrically conductive layer 1b and a dielectric layer 1c as formed one on another in the order mentioned from the inner side to the outer side, and the imaging belt 1 is provided as extending around a plurality of rollers 2, thereby allowing to advance in the direction indicated by the arrow. The electrically conductive layer 1b is connected to ground through a ground brush (not shown) which is disposed to be in sliding contact with a side periphery of the conductive layer 1b. Around the outer periphery of the imaging belt 1 and along the direction of advancement thereof are disposed a charger 3 for uniformly charging the outer peripheral surface of the belt 1 to a predetermined polarity, a recording unit 4 for forming an electrostatic latent image on the belt 1, a developing device 5 for developing the latent image into a visible image, an image transfer device 6 for transferring the developed image to a transfer medium 8 and a cleaning device 7 for cleaning the belt 1. Thus, the outer peripheral surface of the belt 1 defines an imaging surface which is subjected to various image forming processes by the above-mentioned devices while the belt 1 completes one cycle of revolution.
In order to transfer the developed image to a transfer medium in the above-described recording system, the image transfer device 6 comprised of a corona unit applies corona ions having a polarity opposite to the polarity of the developer to a back side of the transfer medium 8 thereby causing the developer defining a desired image to be transferred from the belt 1 to the transfer medium 8. For this purpose, a negative high voltage is applied to the image transfer corona unit 6. During such an image transfer operation, no particular problems arise as long as an electrical resistance of the electrically conductive layer 1b is as small as negligible because the entire electrically conductive layer 1b may be maintained substantially at the ground level at all times thereby allowing to prevent the image forming process from being adversely affected by the operation at the image transfer station.
On the other hand, in the case where the resistance of the conductive layer 1b is appreciably high, there is produced a voltage drop in accordance therewith. A representative relation between sheet resistance .rho..sub.c (resistance value per unit area) of conductive layer 1b and voltage drop .DELTA.V of applied voltage for image transfer is shown in FIG. 2 as a semi-logarithmic graph. If the voltage drop .DELTA.V exceeds a developing threshold voltage, charged toner will be attracted to the imaging surface uniformly, i.e., so-called "blanket development", thereby causing background contamination and a deterioration in image quality.
In order to cope with the above-described situation, the electrically conductive layer 1b may be structured to have sheet resistance .rho..sub.c such that the voltage drop .DELTA.V does not exceed the developing threshold voltage. Supposing that an allowable limit for such a voltage drop .DELTA.V is 10 V, then the corresponding allowable limit for sheet resistance .rho..sub.cL becomes approximately 5.times.10.sup.7 ohms/.quadrature. as indicated in the graph of FIG. 2.
However, even if the electrically conductive layer 1b itself is structured such that its sheet resistance .rho..sub.c is lower than the above-mentioned sheet resistance allowable limit .rho..sub.cL, there is normally formed a locally high electrical resistance region in the vicinity of a seam C which is inevitably formed when fabricating the imaging belt in an endless form unless a sophisticated, expensive fabricating method is used.
FIG. 3 is a graph showing a distribution of resistance to ground R.sub.e of belt 1 along its travelling direction with the abscissa taken for a distance y measured from the seam C along the travelling direction of the belt 1 and the ordinate taken for resistance to ground R.sub.e. It is to be noted that 1.sub.0 indicates a circumferential length of the belt 1. It is seen in the graph of FIG. 3 that the resistance is higher in the vicinity of the seam C and it exceeds the resistance allowable limit R.sub.eL which corresponds to the sheet resistance allowable limit .rho..sub.cL, as indicated by the dotted line. Accordingly, if the image transfer corona unit 6 is on while the seam C and its periphery is moving through a developing station, the before-mentioned "blanket development" would take place at the seam C and its vicinity. This is because, the potential level at the imaging surface of belt 1 fluctuates due to the application of corona ions at the image transfer station while the seam C and its vicinity is passing through the developing station including a developing roller 5a which is maintained at a predetermined developing voltage.